Ultrafast dynamics and phonon softening in Fe_1+<i>y</i>Se_1−<i>x</i>Te_<i>x</i>single crystals

Abstract

The ultrafast quasiparticle dynamics of Fe _1+ _y Se _1− _x Te _x single crystals were investigated by dual-color transient reflectivity measurements (Δ R / R ) from 4.3 to 290 K. The electron–phonon coupling strength λ (=0.16–0.01) and the temperature-dependent energy of longitudinal-acoustic phonons were, respectively, obtained from the relaxation time of a fast component and the period of an oscillation component in Δ R / R . Such a small λ demonstrates the unconventional origin of superconductivity in FeSe. Moreover, the temperature-dependent Δ R / R exhibits anomalous changes at T _c , 90 K and 230 K, unambiguously revealing the phase transition as well as the phonon softening via the magnetoelastic effect.